1. Field of the Invention
Embodiments of the present invention generally relate to the fabrication of semiconductor devices and particularly to the formation of doped regions on a substrate by use of plasma implantation assisted by atomic layer deposition.
2. Description of the Related Art
In the semiconductor fabrication process, it is often necessary to impart impurities into a pure material. Called “doping,” this process invests the material with desirable properties, such as enhanced electrical conductivity. In many processes, it is advantageous to implant various atoms or ions into a semiconductor or semiconductor derivative substrate. For example, boron, phosphorus, and arsenic atoms or ions are routinely implanted into silicon substrates to create “doped” regions to serve as source and drain regions for solid state transistors. In some cases, the substrate is prepared prior to doping by “amorphizing” the region of the substrate to be doped. The crystal structure of the substrate is disrupted by bombardment with silicon, germanium, or argon atoms, creating channels for dopants to penetrate deeper into the substrate. In other applications, nitrogen, oxygen, hydrogen, carbon, fluorine, and various metals, such as indium, antimony, cobalt, and nickel, may be used as dopants to control electrical conductivity or diffusion at interfaces.
Dopants are generally implanted in two ways. In some processes, dopants may be implanted on the surface of a substrate and then heat treated to cause them to diffuse into the substrate. In other processes, dopants may be ionized into a plasma and then driven energetically into the substrate using an electric field. The substrate is then heat treated to normalize distribution of dopants and repair disruption to the crystal structure caused by ions barreling through at high speed. In both types of processes, the heat treatment anneals the substrate, encouraging dopant and ambient atoms located at interstitial positions in the crystal to move to lattice points. This movement “activates” dopants in applications involving control of electrical properties by making the electrical properties of the dopants communicable through the crystal lattice, and it generally strengthens the crystal, which may be important for diffusion control applications.
Even distribution of dopants throughout the target region is generally desired. For applications involving control of electrical conductivity, even distribution of dopants ensures uniform properties throughout the target region. For applications involving control of diffusion, even distribution of dopants ensures no open diffusion pathways for unwanted migration of atoms. For applications involving amorphization, even distribution of dopants ensures uniform density of pathways for subsequent dopants. Heat treatment after implanting promotes even distribution of dopants through the target region.
For more than half a century, the semiconductor industry has followed Moore's Law, which states that the density of transistors on an integrated circuit doubles about every two years. Continued evolution of the industry along this path will require smaller features patterned onto substrates. Stack transistors currently in production have dimensions of 50 to 100 nanometers (nm). The next generation of devices may have dimensions of about 40 nm, and design efforts are being directed toward devices with dimension of 20 nm and smaller. As devices grow smaller, the aspect ratio (ratio of height to width) of features patterned on substrates grows. Devices currently in production may have features with aspect ratio up to about 4:1, but future devices will require aspect ratios potentially up to 100:1 or higher.
Increasing aspect ratios and shrinking devices pose challenges to dopant implantation processes. It is frequently necessary, for example, to implant dopants at the bottom and on the sides of trenches in a field region of a substrate to form features. Energetic implantation processes are directional, with the electric field tending to drive ions in a direction orthogonal to the surface of the substrate. Ions readily impinge on the field region on the substrate, and may penetrate into trenches a short distance, but the electrical bias will drive the ions toward the surface of the field region or side walls of the trenches, preventing them from penetrating to the bottom of the trench. High energy implantation may drive ions to the bottom of the trench, but generally will not achieve conformal implantation and may result in over-implantation in the bottom of the trench and in field areas as compared to side walls.
FIGS. 1A-1D illustrate substrates subjected to conventional implantation techniques. FIG. 1A illustrates substrate 100 featuring field regions surrounding implantation process. A process free of plasma will implant a layer 102 primarily on the field regions, and may implant a layer 104 in the bottoms of the trenches, but any implantation on the side walls will be slow to occur, and layers 102 will grow toward each other as implantation occurs, reducing the opportunity for entry into trenches. FIG. 1C illustrates the implanted layers 102 and 104 after annealing (layers 106 and 108, respectively). Layers 106 feature bulges frequently encountered with conventional implantation, and layers 108 illustrate the tendency of implanted materials to collect in corners. In some processes, the substrate may be rotated to change the angle of incidence, as shown in FIG. 1D, such that the opportunity for precursor materials to penetrate trenches is enhanced. This may result in increased implantation 110 on a portion of sidewall 112. However, any such benefit is minimal, particularly for very high aspect ratio structures, because electric field lines driving the motion of ions are orthogonal to the surface. Thus, stage rotation does not result in conformal implantation or doping.
Therefore, there is a need for a method of conformal doping of high aspect ratio structures on substrates.